1. Field of the Invention
The present invention relates to a NiMnZn-based ferrite which can reduce magnetic loss (core loss) at a high frequency of about 2 MHz or higher, for example, and achieve higher saturated magnetic flux density while forming high sintered density.
2. Related Background Art
Recently, as electronic devices have been reducing their size and weight, switching power supplies have been made smaller. For inductance elements such as transformers and choke coils in switching power supplies, MnZn and MnZnNi ferrites have widely been in use because of their high saturated magnetic flux density.
Such recent switching power supply circuits tend to be switched at high frequencies in order to lower their operating voltage, whereby ferrite cores used therein have been desired to minimize their magnetic loss (core loss) at high frequencies.
The magnetic loss (core loss) of a ferrite core is constituted by hysteresis loss, eddy current loss, and residual loss. The hysteresis loss changes in proportion to the frequency because of DC hysteresis, while the eddy current loss increases in proportion to the square of frequency because of an electromotive force generated by an eddy current caused by an electromagnetic induction action. The residual loss, which is the remainder of loss, is supposed to be caused by so-called domain-wall resonance, natural resonance, and diffusion resonance.
As the prior art that seems to relate to the present application, there are the following literatures. Japanese Patent Application Laid-Open No. 2007-51052 proposes an MnZnNi-based ferrite which can achieve low power loss even at a frequency of 2 MHz or higher. While the above-mentioned ferrite necessitates Ta as an accessory component, the present invention does not necessarily contain Ta.
While Japanese Patent Application Laid-Open No. 2007-51052 suggests an addition of Nb, its range of addition differs from that in the present invention, and they differ from each other in terms of the desirable average crystal particle size to be formed.
Japanese Patent No. 3597666 (Japanese Patent Application Laid-Open No. 10-270231) proposes an MnNi ferrite-based material which is used for a magnetic core of a power transformer and yields low loss at a high frequency of about 1 MHz or higher.
While it suggests an addition of Nb, its range of addition differs from that in the present invention. It does not disclose any desirable average crystal particle size. Its experiments were conducted only at a frequency of 1 MHz; no experiments were carried out for seeing effects at 2 MHz.
Japanese Patent Application Laid-Open No. 10-326706 proposes a ferrite material having high sintered density and high strength while exhibiting low loss in a high-frequency region of about 1 MHz or higher.
The addition of Nb is neither disclosed nor suggested. There is no disclosure concerning the desirable average crystal particle size to be formed. Its experiments were conducted only at a frequency of 1 MHz; no experiments were carried out for seeing effects at 2 MHz.
Japanese Patent No. 3917216 (Japanese Patent Application Laid-Open No. 10-64715) proposes a ferrite magnetic core material having high saturated magnetic flux density with low loss, which is employed in power transformers and the like in switching power supplies and the like.
While it suggests an addition of Nb, its range of addition differs from that in the present invention. There is no disclosure concerning the desirable average crystal particle size to be formed. Its experiments were conducted only at a frequency of 100 kHz; no experiments were carried out for seeing effects at 2 MHz.
Japanese Patent No. 3597673 (Japanese Patent Application Laid-Open No. 11-3813) proposes a ferrite material yielding low loss in a high-frequency region of about 1 MHz or higher.
While it suggests an addition of Nb, its range of addition differs from that in the present invention. There is no disclosure concerning the desirable average crystal particle size to be formed. Its experiments were conducted only at a frequency of 1 MHz; no experiments were carried out for seeing effects at 2 MHz.
Japanese Patent Application Laid-Open No. 7-211533 proposes a method of manufacturing an oxide magnetic material yielding low loss at high frequencies and low residual magnetic flux density, which is used in switching power supplies and the like.
It does not relates to MnZnNi ferrites but MnZn ferrites (see comparative examples where Ni=0 in the present application).
Its experiments were conducted only at frequencies of 500 kHz and 1 MHz; no experiments were carried out for seeing effects at 2 MHz.
Under these circumstances, it has been desired to propose a ferrite applicable to higher frequencies, a novel NiMnZn-based ferrite which can reduce the magnetic loss (core loss) at a high frequency of 2 MHz or higher and achieve higher saturated magnetic flux density while forming high sintered density in particular.